Added barShadowEnabled property and improved performance of the shadow

Hey,

Awesome library, I've been looking for a decent graph pod for a while now so thanks for open sourcing.

While playing with it I noticed that the performance of the BarGraph was a little suboptimal due to the shadow on the bar's layers. A quick fix later (don't forget to set layer.shadowPath when you use shadows!) and it's solid 60fps.

I've also added a toggle to allow people to enable/disable the shadows at will - as I didn't actually need them.

Thanks,
Will